1. Field of the Invention
The invention relates to a method and apparatus employing one or more miniature gas containers, for producing low concentration gases for testing gas detection instruments.
2. Description of Related Art
Gas detection instruments are an important category of electronic devices. They are used to safeguard human lives and property in mines, confined spaces, chemical plants and places where combustible gases (CH4, etc.) or toxic gases (CO, H2S, SO2, NOx, Cl2, etc.) are present, or oxygen deficiency often occurs. Gas detection instruments are also widely used in domestic applications. For example, many homes have at least one detector monitoring concentrations of carbon monoxide, which is often produced by faulty furnaces or poorly ventilated fireplaces. The heart of each instrument is at least one sensor that converts chemical energy to electrical energy. The most common sensor types include metal oxide semiconductor (MOS) sensors, calorimetric sensors, thermal conductivity sensors, catalytic combustion sensors, electrochemical sensors, infrared sensors, and photo ionization and flame ionization detectors. Although most sensors possess good specificity, they are subject to changes in gas sensitivities. For example, a Galvanic type oxygen sensor can fail suddenly if the capillary hole, through which oxygen diffuses into the sensor cell, is blocked by water or liquid electrolyte. Catalytic combustion sensors that are used for detecting combustible gases can lose sensitivity completely after exposure to silicones, which form a coating on the catalyst, blocking gas reactions. The use of a defective, low sensitivity instrument is extremely dangerous as it often gives a false or misleading reading, putting human lives and property in great danger.
Gas detection instruments are required to be calibrated on a regular basis, usually monthly. Regular calibrations ensure accuracy of the instrument immediately after calibration. Since sensors can, however, fail between calibrations, users therefore need to verify the condition of their instrument prior to each use. This is a functional test done by briefly applying to the instrument a gas of known concentration and verifying instrument display and alarm operation; such a test is generally referred to as a bump test. If the instrument response is prompt and the output is within a pre-determined percentage window with reference to the test gas concentration, and the alarm devices operate as expected, the instrument is considered to be working properly. Otherwise the instrument fails the test and will need to be recalibrated and/or fully examined.
At the present time, instrument users rely on pressurized, premixed gas bottles for bump testing their instruments. A regulator is used to release the gas from the bottle and a mechanical adaptor is used to direct the gas flow to the sensor window of the instrument. Generally speaking, premixed gas bottles are large in size and inconvenient to carry in the field. The commonly used bottles are 1.2 ft3 steel and 2 ft3 aluminum with an internal gas pressure of approximately 500 psi. The gas volume is limited and the cost of each test is high. Moreover, the high pressure can cause injury if not handled properly or the bottle is exposed to excessively high temperatures.
In order to improve portability, an apparatus is available from Draeger Safety AG & Co. of Germany for conducting testing and calibration by using a 1 liter empty bottle, called a calibration bottle, and an adaptor to connect the bottle to the instrument. A gas ampoule made of glass is shattered inside the calibration bottle to allow gas to be diluted by air to a desired concentration, and the diluted gas is then introduced to the instrument through the adaptor. The apparatus has several disadvantages in that the user has to dispose broken pieces of glass every time after test and the calibration bottle is still large in size and inconvenient to carry daily in most workplaces.
Efforts have been made to generate gases onsite for testing and calibrating gas detection instruments. For example, U.S. Pat. No. 5,395,501 discloses an electrochemical generator for generating H2S, H2 and Cl2, the generator passing an electric current through an electrolytic cell to generate the gas, which is then diluted into a carrier gas stream. U.S. Pat. No. 6,234,001 discloses an apparatus for generating a reference gas by passing a stream of carrier gas through a chamber containing a volatile liquid. As the stream passes through the chamber, the volatile liquid evaporates into a gas that blends into the carrier gas to form the reference gas.
Gas generators are available for a very limited number of gases. They are usually bulky, position sensitive, and not suitable for carrying in the field.
Due to the cost and availability, household products have been used to test equipment. Examples of this are use of gasoline vapor to test a combustible gas detector and use of cigarette smoke to test a carbon monoxide detector. Such tests are not good practice as the test gas concentrations are so high that even a badly degraded instrument may show a response, while in reality a much lower gas concentration is harmful, deadly and/or explosive.